Phased introduction of lithium into the pre-lithiated anode of a lithium ion electrochemical cell

ABSTRACT

The present invention relates to a method for combining anode pre-lithiation, limited-voltage formation cycles, and accelerating aging via heated storage to maximize specific capacity, volumetric capacity density and capacity retention of a lithium-ion electrochemical cell.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/685,550, filed on Aug. 24, 2017, which is a continuation of U.S.application Ser. No. 14/167,076, filed on Jan. 29, 2014, which claimsthe benefit of U.S. Provisional Application No. 61/758,481, filed onJan. 30, 2013. The entire teachings of the above applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

In the course of assembling and operating a lithium ion cell, variousprocess steps are usually performed to optimize performance: 1)assembly; 2) vacuum drying; 3) electrolyte and protective additivesfilling; 4) sealing; 5) ambient aging; 6) formation cycling; 7) elevatedtemperature aging; 8) degassing and re-sealing; and 9) performancechecking. There may be other process steps, but these may be consideredto be the relevant ones. Assembly generally includes the mounting of atleast one anode, one cathode, one separator and at least two contactleads into a pouch, a can, a button cell, or other gas tight enclosure.Vacuum drying generally includes an application of vacuum and heat priorto electrolyte filling. This process may last from about 12 hours toabout three days. Electrolyte filling generally can be performed byinjecting an electrolyte mixture into a vacuum dried pouch assembly andthen vacuum sealing the pouch. Ambient aging generally allows the vacuumsealed cell to fully adsorb electrolyte prior to cycling for the firsttime. Formation cycling occurs by charging the completed cell at a lowrate, usually over a 12 hour or longer period, in order to form a solidelectrolyte interphase (SEI) or passivation layer primarily on the anodesurfaces. These layers passivate the lithium active surfaces againstadditional reactions. A large amount of lithium can be lost in the firstformation cycle (5 to 30% of initial capacity depending on anode type),but additional losses can continue to occur. Such ongoing losses areoften significant, and may be up to, or greater than an additional 20%throughout customer cycling. For some customer purposes, elevatedtemperature aging is usually used to pre-age the cells so that theremaining cycles are more stable from the first customer cycle to the200^(th) customer cycle. The elevated temperature aging step istypically performed at 50 to 60 degrees Centigrade, and may last for upto a week. During this step, additional lithium is lost along with theconsumption of moisture molecules and electrolyte. The cell is thenopened, degassed, and then resealed under vacuum conditions. After thesesteps are completed, the cell is ready for performance tests includinginitial capacity and capacity retention. Performance checks are made bycycling the cell at a prescribed rate and the cells are sorted for salecategories.

The losses of lithium can be categorized: (1) formation cycle buildingof SEI layers (primarily on the anode) by decomposition of theelectrolyte; (2) the reduction of water molecules left over from thevacuum drying process and by diffusion through the package walls andseals; and (3) rebuilding of the SEI layers required due to theexpansion and contraction of the active material layers (primarily inthe anode). In standard (non-prelithiated) lithium ion cells, lithium issupplied by the cathode during the first charging cycle, and somecathode material forever becomes inactive as less lithium is returned tothe cathode on subsequent charge cycles. This unused cathode materialbecomes “dead weight”. Any additional loss of lithium will furthersubtract directly from specific capacity. Lithium can be added to thecell prior to assembly as described by U.S. patent application Ser. No.13/688,912, which is incorporated herein by reference, to replace firstcycle losses. The amount of pre-lithiation is usually selected to avoidformation of lithium metal or dendrites on the anode; maximum anodecapacity cannot be exceeded during any charge cycle, and particularlynot during the initial charge cycle. There is a need to extend thecathode capacity available for cycling and maximize the specificcapacity for the target number of customer cycles.

The depletion of electrolyte occurs during: 1) The formation cycle andformation of initial SEI layers, primarily on the anode; 2) The elevatedtemperature aging cycle where additional electrolyte is consumed; and 3)Customer cycling. Reducing these losses could increase cell lifetime.Reducing electrolyte consumption could stabilize cell resistance andimprove capacity retention. Reducing the consumption of electrolyteadditives could reduce cell cost.

OBJECT OF THE INVENTION

It is the object of this invention to increase or improve the specificcapacity, volumetric capacity density and capacity retention (asdescribed by the retention rate measured between the first and n^(th)customer cycle) in a lithium ion cell with the use of the phasedintroduction of lithium into pre-lithiated anodes of lithium ion cellsequence.

SUMMARY OF THE INVENTION

The invention relates to processes for lithiating and/or charging alithium ion cell and to the cells produced by the processes. Heat and orpartial formation cycling are used to accelerate and control lithiumlosses occurring in the early operation of a lithium-ion battery. Theprocedure described below combines anode pre-lithiation with controlledlithium loss generation and controlled transfer of lithium betweencathode and anode in order to maximize cell cycling capability andretention.

In one embodiment, the process includes the steps of: a) pre-lithiatingan anode; b) assembling the anode, a cathode, a separator andelectrolyte into a sealed cell; c) charging, preferably partiallycharging, the cell; d) optionally discharging the cell and repeatingstep c), possibly at incrementally higher voltages to stimulate furtherSEI loss; e) optionally applying an elevated temperature for ½ to 7 ormore days; f) optionally discharging the cell; and g) charging the cellto the normal full voltage.

In accordance with a preferred embodiment of the invention, the anode ispre-lithiated. The invention can use commercially availablepre-lithiated anodes or can include the step of pre-lithiating theanode. Preferably, the anode is pre-lithiated in accordance with theprocesses described in U.S. Ser. No. 13/688,912, which is incorporatedherein by reference. Alternately, the anode can be pre-lithiated usinglithium bearing additives or lithium bearing powders. The pre-lithiatingamount is preferably calculated to be approximately equal (+/−10%) tothe expected initial formation cycle losses and the elevated temperaturestorage accelerated aging step losses. If the cathode to anode loadingbalance allows, the more preferred pre-lithiating amount is greater thanthe amount needed to offset formation and aging losses, in order toleave a reservoir of lithium to improve retention during customercycling. Half-cell measurements can be useful to estimate the reversiblecapacities of the anode and cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 . Illustration of lithium movement across anode and cathodeduring phased introduction of lithium into pre-lithiated anodes oflithium ion cell. Percentages are relative to maximum lithium capacityof each electrode.

FIG. 2 . Comparison of cells. The cell shown by the upper lines in eachgraph uses the phased introduction of lithium into pre-lithiated anodesof lithium ion cell. The lower line in each graph represents a controlcell.

DETAILED DESCRIPTION OF THE INVENTION Example Pre-Lithiation Calculation

Cathode reversible capacity, LiCoO₂ C_(r) = 3.2 mAhr/cm² Anode maximumcapacity (100% in FIG. 1) A_(m) = 3.5 mAhr/cm² Pre-lithiation amount P =1.5 mAhr/cm² Formation capacity loss F = 0.7 mAhr/cm² Elevatedtemperature age loss E = 0.8 mAhr/cm² Irreversible cathode Li⁺ donationI_(c) = 0.0 mAhr/cm²

Before the pre-lithiation amount is determined, full cell capacityvalues of F and E are measured for the intended anode and cathodecombination. The pre-lithiation amount added is preferably in accordancewith the formula: P≥F+E−I_(c), subject to the constraint that the totalamount of lithium residing in the anode must be always less than A_(m).For example, in the cell of Example 1, A_(m) was measured to be 3.5mAh/cm², F was measured to be 0.7 mAh/cm²; E was measured to be 0.8mAh/cm², I_(c) was estimated to be 0.0 mAh/cm², and P was determined tobe between 1.5 and 3.5 mAhrs/cm².

Pre-lithiation is understood here to mean lithium added to the anodeprior to cell assembly. Other sources of lithium include those of thecathode. A highest reversible lithium capacity for an anode can beachieved by the phased introduction of lithium by pre-lithiation,forming, aging the cell, and final introduction of lithium from thecathode by the full voltage charging step. By following this sequence,anode reversible capacity is optimized while never violating the maximumlithium content where dendrites would form. FIG. 2 indicates thecapacities and retention characteristics of a cell processed with thenew pre-lithiated sequence and a control. The button cell used here forexample is composed of a graphite anode and a lithium cobalt oxidecathode. For other types of cathodes and anodes, the degree ofimprovement varies with the initial and elevated temperature losses inaccordance with the calculations shown above. Some cathodes are designedto give extra lithium during the first charge irreversibly but thisstrategy leaves additional dead weight as a result. For cells with suchcathodes, the method described herein of limiting the first cycle chargecan also benefit the final cell capacity while protecting the anode fromviolating the maximum lithium content limit. Most cathodes have lowlevels of irreversible and reversible losses. Some anodes have lowinitial losses but these types of anodes typically have low ratecapability. In general, this phased introduction of lithium is mostvaluable in cases where significant losses would otherwise take place.

The anode typically comprises a compatible anodic material which is amaterial which functions as an anode in an electrolytic cell. The termanode is intended to include negative electrodes, conductive foils,anode sheets, anode substrates, or non-reactive plating-capable foils.In one embodiment, anodes are lithium-intercalating anodes. Examples ofmaterials that comprise lithium-intercalating anodes include but are notlimited to carbon, graphite, tin oxide, silicon, silicon oxide,polyvinylidene difluoride (PVDF) binder, and mixtures thereof. In afurther embodiment, lithium-intercalating anode materials are selectedfrom graphite, cokes, mesocarbons, carbon nanowires, carbon fibers,silicon nanoparticles or other metal nanomaterials and mixtures thereof.In another embodiment, alloying metals such as tin or aluminum may beused to host the lithium metal.

During the pre-lithiation step, a reducing current is applied to theanode in such a way as to intercalate (or otherwise host) the lithium.The anode is bathed in a solution comprising a non-aqueous solvent andat least one dissolved lithium salt, preferably LiCl. The termnon-aqueous solvent is a low molecular weight organic solvent added toan electrolyte which serves the purpose of solvating the inorganic Lisalt. Typical examples of a non-aqueous solvents are butylene carbonate,propylene carbonate, ethylene carbonate, vinylene carbonate, vinylethylene carbonate, dimethyl carbonate, diethyl carbonate, dipropylcarbonate, methyl ethyl carbonate, acetonitrile, gamma-butyrolactone,triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, roomtemperature ionic liquids (RTIL) and mixtures thereof. In oneembodiment, a non-aqueous solvent is selected from ethylene carbonate,vinylene carbonate, vinyl ethylene carbonate, gamma-butyrolactone, andmixtures thereof. In a second embodiment, a non-aqueous solvent isgamma-butyrolactone. In a third embodiment, an additive can beintroduced to support high quality SEI formation. The additive could bevinylene carbonate, ethylene carbonate, fluorinated ethylene carbonate,or maleic anhydride. In a fourth embodiment, a gas such as CO₂ or SO₂ issparged into the non-aqueous solution in order to: increase saltsolubility; increase the ionic conductivity; support the formation ofLi₂CO₃ or Li₂SO₃ SEI layer; and increase the lithiation efficiency.

Inexpensive salts with gaseous decomposition products can be halidessuch as LiCl, LiBr, and LiF. LiCl and other simple salts can bedifficult to dissolve or ionize in non-aqueous solvents. Solvents suchas propylene carbonate (PC), dimethyl carbonate (DMC), and acetonitrilesupport only trace amounts of LiCl in solution without the use of acomplexing agent such as AlCl₃. AlCl₃ and other complexing agents can bedifficult to handle in regard to moisture management and highcorrosivity. In addition, some solvents that can dissolve halide salts,such as DMSO or tetrahydrofuran (THF), do not allow complete ionizationof the salt, and/or attack the binding polymers in the anode composites.Gamma-butyrolactone has been found to facilitate the dissolution andionization of the desirable alkali metal halide salts. It combines goodsolubility of the alkali metal halide salts with compatibility with TFETeflon_(c), PVDF, butadiene rubber and other binders. The use of halidesalts with gaseous decomposition products such as LiCl minimizes theproduction of solid precipitates during the lithiation process. Sincethe lithiation process products are primarily lithium ions and gas,there are few solid precipitates or intermediate compounds that canaccumulate in the non-aqueous solvent solution. Removal of dissolved gasfrom the non-aqueous solvent solution is preferred over solidprecipitates during long term continuous operation of a productionsystem.

Gamma-butyrolactone also has a capable electrochemical window, includingthe lithium potential near −3 volts vs. a standard hydrogen electrode(SHE). It is a capable electrolyte with high permittivity and lowfreezing point, and can dissolve and ionize up to a 1 M concentration ofLiCl. A modest amount of heat can be used to reach this value. In oneembodiment, the heat to dissolve and ionize up to a 1 M concentration ofLiCl is between about 20° C. and 65° C., such as between 30° C. and 65°C., such as between 38° C. and 55° C. In a more preferred embodiment,the heat is between about 25° C. and 55° C. In a most preferredembodiment, the heat is about 25° C. The lithiation tank can also havean internal circulating pump and distribution manifold to preventlocalized salt concentration deprivation.

Dissolved gas such as CO₂ or SO₂ can enhance the lithiation process. Itincreases the solubility of the salt, the ionic conductivity of thenon-aqueous solvent, and doubles the efficiency of lithiation. Since CO₂is inexpensive, easily dried, chemically safe, and a potential buildingblock gas for a high quality SEI layer, it has been selected as thepreferred dissolved gas. CO₂ preferentially reacts with trace H₂O andLi′ during the lithiation process to form a stable, insoluble SEImaterial (Li₂O, Li₂CO₃ etc.). The moisture level in the lithiation tankis driven down by the consumption of CO₂ and H₂O according to thisprocess, and care is given to control the moisture level in the tank tobetween about 5 to 20 ppm. In this way, anode lithiation with a qualitySEI material is produced continuously.

The intercalation or plating process for lithium ions (or generallylithiation) from 1 or 0.5 M LiCl salt, for example, ingamma-butyrolactone solvent will occur at about 4.1 volts measuredbetween the anode sheet and the reference electrode up to a reducingcurrent density of 2 mA/cm² or more. As intercalation rates areincreased too far beyond this current density, dendrites or lithiumplating may begin to take place which harm the final battery orelectrochemical cell performance. This current density limit will varydepending on the graphite or other anode material porosity etc. In orderto control both the currents and dependant voltages accurately, it maybe necessary to divide the field plate into zones. Other metals can alsobe plated or intercalated with this method including sodium as anexample. As mentioned above, the byproduct of the intercalation processwhen using a halide alkali metal salt is an evolving gas at the counterelectrode (field plate). In a preferred embodiment, the evolving gas isselected from F₂, Cl₂, Br₂, and mixtures thereof. In a more preferredembodiment, the evolving gas is Cl₂.

Prior to entering the lithiation bath, the anode material can bepre-soaked in an electrolyte solution. The pre-soaking of the anodematerial will ensure full wetting of the material prior to the start ofthe lithiation process. This pre-soak bath can contain a non-aqueoussolvent with or without a lithium salt, with or without a sparge gas,and with or without an SEI promoting additive.

The evolution of gas at the field plate or counter electrode can resultin evolving gas entering into, and/or being released from, the bathsolution. As a result, controlling the build-up of dissolved andreleased gas is desired to avoid corrosion, as for example, in thehypothetical case of trace water contamination reacting with chlorinegas, to form HCl during chlorine gas evolution. The tank assembly can beconfigured to control the introduction of moisture into the system byusing a dry gas blanket on top of the liquid. In one embodiment, the drygas (1-10 ppm moisture) is selected from helium (He), neon (Ne), argon(Ar), krypton (Kr), xenon (Xe), sulfur hexafluoride (SF₆), nitrogen(N₂), dry air, carbon dioxide (CO₂) and mixtures thereof. In a preferredembodiment, the dry gas is selected from nitrogen, argon, carbondioxide, dry air and mixtures thereof. Moisture ingress can also becontrolled by having a long narrow gap entry and exit tunnel for theanode film where a counter flowing dry gas is used to mitigate air entryinto the system.

The process and apparatus can preferably continuously control moisture,gas, and small quantities of lithiated organic compounds during acontinuous lithiation process. Liquid can be drawn from a bath through aseries of valves. The liquid can be delivered in a batch mode to arefluxing unit, or it can be continuously circulated through aconditioning loop including distillation or reverse osmosis. The refluxunit can take batches of material through a vacuum refluxing processthat will remove both accumulated gas as well as moisture from theliquid. In one embodiment, the accumulated gas is selected from F₂, Cl₂,Br₂, and mixtures thereof. In a more preferred embodiment, theaccumulated gas is Cl₂. The use of reflux conditioning instead of adistillation process can prevent a change in the salt concentration ofthe working fluid which would result in a loss of salt content throughprecipitation. Once the batch liquid has been refluxed for a designatedperiod of time, the liquid can be returned to the bath with a lowermoisture and gas content. The size and rate of the reflux unit can bematched to the moisture ingress rate and to the gas production rate inorder keep the bath liquid at optimum conditions. The reflux rate can beincreased through use of multiple simultaneous batches and through theuse of high rate reflux equipment such as a rotary evaporator and highvacuum conditions. The reflux batch moisture content typically decays inan exponential fashion and the turnover rate can be tuned for optimalmoisture control with minimal energy input and equipment cost.

The refluxing unit can be placed after a salt dosing unit. The saltdosing unit can be used to add and mix the desired salt into thenon-aqueous solvent solution. The temperature of the dosing unit can beheld to maximize the solubility of the salt in the electrolyte and theelevated temperature can also be used as a pre-heating step for therefluxing unit. In one embodiment, the dosing unit maintains an elevatedprocess temperature of between about 20° C. and 65° C., such as between30° C. and 65° C. or 38° C. and 55° C. In a more preferred embodiment,the dosing unit maintains an elevated process temperature of betweenabout 25° C. and 55° C. In a most preferred embodiment, the dosing unitmaintains an elevated process temperature of about 25° C. The benefit ofdosing in the salt in a dosing unit before the refluxing unit is thatthe salt does not have to be in a completely dry state. Removing themoisture from a solid phase salt can be very difficult. Once a salt isdissolved into solution, however, the water content of the salt can beremoved through the refluxing process. Maintaining the dosing unit at anelevated temperature increases the solubility of the lithium salt in thenon-aqueous solvent and ensures full dissolution of the salt prior tothe refluxing unit.

The conditioning/replenishment loop operates in a continuous mode andcan also be used to remove dissolved gases from the bath liquid throughuse of a membrane contactor. The gas output from the membrane contactorand the reflux unit can be passed through a scrubber to capture anyeffluent, such as chlorine gas, produced by the process. In oneembodiment, the dissolved gases are selected from F₂, Cl₂, Br₂, andmixtures thereof. In a more preferred embodiment, the dissolved gas isCl₂. The bath liquid can also be paired against either vacuum or a drygas within the membrane contactor in order to remove unwanted gases. Inone embodiment, the dry gas is selected from helium (He), neon (Ne),argon (Ar), krypton (Kr), xenon (Xe), sulfur hexafluoride (SF₆) nitrogen(N₂), carbon dioxide (CO₂), dry air and mixtures thereof. In a preferredembodiment, the dry gas is selected from nitrogen, argon, carbondioxide, dry air and mixtures thereof.

An inline heater/chiller can be used to maintain a desired tanktemperature to maintain consistent bath operating conditions, even withvariations in facility temperature. Controlled lithiation tanktemperatures can aid in the formation of a high quality SEI layer. Inone embodiment, the inline heater/chiller maintains a tank temperatureof between about 20° C. and 55° C. In a more preferred embodiment, theinline heater/chiller maintains a tank temperature of between about 20°C. and 30° C. In a most preferred embodiment, the inline heatermaintains an elevated tank temperature of about 25° C.

A filter unit can be used to remove any accumulated particulatecontamination. The filter unit can be located at various points in theloop including prior to the pump and after the salt dosing unit. Thefilter unit can be used to remove particulates from the non-aqueoussolvent in cases where a non-halide lithium salt such as LiNO₃ is usedsuch that a precipitate may be formed at the field plates.

Lithium halide salt can be added to the non-aqueous solvent using thesalt dosing unit. An excess of solid lithium salt can be maintainedwithin the dosing unit to keep the lithium salt concentration within theloop and within the bath at the desired level (i.e., a saturatedsolution of about 0.5 M to 1.0 M) over long periods of time. The dosingunit can be configured to keep the solid salt from entering the bath orrefluxing unit. By dosing salt prior to the refluxing unit, there is noneed to separately dry the salt with its high water binding energy inits granular state. In one embodiment, the lithium salt within the saltdosing unit is selected from LiF, LiCl, LiBr, and mixtures thereof. In apreferred embodiment, the lithium halide salt within the salt dosingunit is LiCl. Dissolved lithium salts can be carried through the rest ofthe loop. The fluid circulation loop pump rate can be matched tomaintain a constant lithium salt concentration in the tank. For a givenanode substrate process rate, a matching loop circulation rate will dosethe same amount of lithium salt as the lithiation process consumes. Asthe anode process rate is increased or decreased, the loop circulationrate can be modified to maintain an equilibrium state within the bath.

Depending on the specific tank conditions, the bath fluid can be treatedusing a circulating loop, a refluxing unit or a distillation unit. Acirculating loop can dose in salt, remove dissolved gases, control thebath temperature and remove particulate contaminants. A refluxing unitis effective at removing dissolved gases and for removing moisturecontent without reducing the salt content of the solution. Adistillation unit is effective at removing dissolved gases, removingmoisture content, removing all salt content and removing lithiatedorganic compounds. The output from the distillation unit can be fed backinto a dosing and refluxing unit to reestablish the salt content ifrequired. The effluent from the distillation unit can be collected andtreated to recover used salt for reuse in the lithiation process. Forexample, DMC solvent will rinse away all but the insoluble salt so thatthe salt may be re-introduced into the dosing unit. Recirculating loops,refluxing unit and distillation units can be shared across multipletanks that have different input and output requirements as a means ofminimizing equipment size and cost.

When the anode is lithiated to the extent of the irreversible andextended cyclic loss amount, as well as the intended cycling amount, itcan be assembled into a battery or electrochemical cell with a cathodematerial that does not initially contain lithium. This type of cathodematerial can be much less expensive than lithium containing cathodematerials, and examples include, but are not limited to, MnO₂, V₂O₅ andpolyaniline. The cost of the battery or cell produced with this methodwill be lower due to the lower cost of the feedstock lithium salt.Alternatively, the cathode can contain lithium.

The anode, cathode and separator are then assembled into a cell housing,such as a button cell housing. The cell is preferably vacuum dried.

Electrolyte is added, and the cell is sealed, preferably during anapplied vacuum. Preferred electrolytes include EC/DMC/DEC and 1M LiPF₆and 1% VC. The cell is then sealed (e.g., vacuum sealed) and preferablystored at ambient temperature (between about 15 and 30° C., preferablyabout 20° C.) for 6 to 24 hours, preferably between 12 and 13 hours, toallow for electrolyte adsorption and swelling.

The first formation cycle charge step can be performed to a voltageabove that of electrolyte reduction (typically around 3.7 volts for acarbonate based electrolyte system, such as 1/1/1 EC/DMC/DEC and 1MLiPF₆ and 1% VC with graphite anodes) but below that voltage where aresulting lithium dosage would be higher than the anode's maximumlithium capacity or that would result in dendrite formation. The anode'smaximum lithium capacity can be measured in half-cell. The appropriateformation cycle charge voltage is determined empirically by measuringcapacity, and insuring that the anode's maximum lithium handlingcapacity is not exceeded. This step typically results in, or stimulates,a partial formation of the SEI layer. In a preferred embodiment,approximately 90% of the SEI layer is formed.

The formation cycle, or charging step, above can optionally be repeatedone or more times to further complete the formation of the SEI. Thepreferred SEI layer is judged to be more complete when subsequent cyclesexhibit low capacity loss, indicating the low lithium loss or highcapacity retention. Optionally, the formation cycle(s) can be performedat an elevated temperature such as 50 degrees Centigrade to combine thebenefits of the formation cycle with that of elevated temperature agingmentioned below.

After the formation cycles are completed, the cell is optionally left inthe charged or partially charged state and aged under heat for 12 hoursto one week (or more) between 25° C. and about 60° C. or higher.Preferably, the aging step is performed between 1 to 10 days, morepreferably 3 days. Higher temperatures may be used if they are notdetrimental to the cell. After the cell is aged and cooled back toambient conditions, the cell is optionally discharged.

The cell can then be charged to the desired charging voltage for thecathode-anode system or cell. For example, in the graphite anode andLiCoO₂ cathode system, the charging voltage is typically 4.2 volts.

The cell is now ready to be discharged for performance testing. At thispoint, the anode lithium losses due to the formation cycles and theaccelerated aging have occurred and have been compensated by thepre-lithiated lithium dosage, without exceeding the anode's lithiumloading limit and avoiding dendrite formation (see FIG. 2 ). When thecell is now cycled, the cathode's reversible lithium amount can becycled and little dead weight is left in the cathode (a small value isshown in the diagram for cathode presence only). A similar cathodecapacity is exhibited in the half cell where no limitation of lithium ispresent. Optionally, the cell can be opened and vacuum resealed at thistime.

By managing the source and timing of the lithium introduction in thisway, the lithium ion cell can be safely optimized for specific andvolumetric capacity and capacity retention. Optionally, the formationcycle can be performed at an elevated temperature such as around 35° C.to 50° C. in order to accelerate losses and possibly improve the natureof the SEI material.

This method is different from the alternative assembly techniques forboth standard (non-prelithiated) and pre-lithiated anodes. In a standardanode condition, the mass of the cathode is selected to prevent theplating of lithium (Li) on the anode during the first charge afteraccounting for lithium losses due to the initial formation cyclecapacity loss. Secondary losses due to elevated temperature storage andcustomer cycling directly impact cell capacity. If the initial cathodeis too large, then lithium will plate onto the anode during the firstformation cycle which will cause dendrites and will lead to early cellfailure due to shorting between the anode and cathode. If the initialcathode is too small, then the cell specific capacity will be lower. Inboth of these cases, there will be “dead weight” associated with unusedcathode capacity after the first formation cycle and this “dead weight”will increase during heated temperature storage and customer cycling.

If a pre-lithiated anode is charged against the matching cathode, butwithout one or more capacity-limited initial formation cycles, eitherthere will be dendrites or a lost opportunity to support full cathodecapacity with maximum capacity retention. Compensating for SEI loss onlyis still an improvement over the standard non-prelithiated anodesituation, but the lithium losses that occur during the heated storagestep and or subsequent cycling cannot be recovered.

The combination of pre-lithiating an anode, limiting initial formationcycles, and accelerating aging via heated storage and or heatedformation allows for the maximum addition of lithium to compensate forSEI and other lithium losses through the entire manufacturing process.This results in higher specific and volumetric capacity and capacityretention (See FIG. 2 ).

In addition to a single limited formation step (charge step) of a cellprior to the optional elevated temperature storage, a series of limitedincremental formation steps may also be performed, possibly to varyingcell voltages such as an initial charge to 3.7 V, followed by adischarge, then a secondary charge to 3.8 V.

Example

The following is a detailed example of an anode preparation andprocessing. 25 micron thick copper foil was cleaned with isopropylalcohol and Kimberly-Clark Kimwipes to remove oil and debris and thendried in air. A solution was prepared by adding 2.1 grams of 1,000,000weight PVDF powder from Arkema Fluoropolymers Div. to 95 ml of dry NMPsolvent from Aldrich Chemical. The solution was mixed with a stir barovernight to fully dissolve the PVDF material. The solution was kept inthe dark to prevent the light sensitive solvent from reacting. 33.9 mlof this PVDF solution was then added to 15 grams of Conoco PhilipsCPreme G5 graphite and 0.33 grams of acetylene black and stirred for 2hours in a ball mill at 600 RPM with a single ⅜″ diameter stainlesssteel ball. The resulting slurry was cast onto the copper foil using avacuum hold down plate with heating capability. The finished graphitethickness after casting and drying at 120° C. was about 100 microns or14 mg/cm². The anode sheet was then die punched into 15 mm diameterdiscs and then pressed at about 3000 psi and 120° C. for use in a 2032button cell assembly. The copper/graphite anode discs were then vacuumbaked at 125° C. and about 1 mTorr in a National Appliance Company model5851 vacuum oven for at least 12 hours.

The anode discs were then transferred into a Terra Universal dry airglove box with −65° C. dew point air supplied by compressed dry airpassed through a Kaeser two stage regenerative drier. The anode discswere then vacuum infiltrated with a GBL solvent with a 0.5 Mconcentration of LiCl salt solution. This electrolyte solution had beenprepared by heating to 90° C. and then vacuum refluxing at about 1 mTorrfor 6 hours to remove moisture down to about 10 ppm. The anode discswere allowed to soak for a half hour at vacuum conditions, a half hourin atmospheric pressure conditions and a half hour in the lithiationvessel itself prior to any currents being passed. The lithiation vesselincluded a constant bubbling of CO₂ gas to achieve a saturation leveland a temperature of 30° C. Test leads from a Maccor 4300 battery testerwere connected to the anode sample (black working) and glassy carbon(red counter) electrode. Voltage at the working electrode is monitoredvia an Ag/AgNO₃ non-aqueous reference electrode. A reducing current of 2mA/cm² was applied to the graphite anode until a total of 1.5 mAhr/cm²was achieved. The pre-lithiated anode disc was then rinsed in puredistilled GBL and vacuum dried. The anode discs were then assembledagainst either LiFePO₄ or LiCoO₂ 12 mm diameter cathode discs. Theseparator used was Celguard 2400, and about 0.2 ml of electrolyte wasused in the assembly. The electrolyte was 1:1:1 EC:DMC:DEC with 1M LiPF₆salt and 1% VC with moisture levels at about 10 ppm. A vacuum wasapplied to the assembled cell to remove bubbles before venting with adry gas and crimping in an MTI model MT-160D crimping tool. Thegraphite/lithium cobalt oxide button cell was connected to the MaccorSeries 4000 Battery Test system for processing as follows: 1) A firstcharge was applied to the cell by a constant current of 0.25 mA/cm² to avoltage of 3.7 in order to bring 1.7 mAhr/cm² additional lithium intothe anode. The total amount of lithium dosed (1.5 mAhr/cm² frompre-lithiation and 1.7 mAhr/cm² from the cathode) into the anode wassafely below the maximum amount of 3.5 mAhr/cm². At this voltage,significant SEI formation has taken place and about 0.7 mAhr equivalentlithium has been consumed. This is reflected in FIG. 1 as AfterFormation/Limited Charge Cycle. 2) The cell was then subjected to anelevated temperature of about 50° C. for 3 days in order to acceleratethe aging that usually takes place within the first 200 or so customercycles. This accounts for an additional 0.8 mAhr/cm² equivalent oflithium loss. FIG. 1 represents this step as After Temperature AgingCycle. 3) The cell is now reconnected to the Maccor 4000 test system atroom temperature and charged up to the normal voltage for the cathodesystem at a C/3 rate. The voltage setting was 4.2V for the charge and 2Vfor the discharge and the current was 1 mA/cm². This is represented bythe After Full Cathode Charge in FIG. 1 . 4) The cell was thendischarged and charged for performance tests with results shown in FIG.2 .

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed:
 1. A process of maximizing specific capacity andcapacity retention of a lithium ion cell comprising: a) pre-lithiatingan anode; b) assembling the anode, a cathode, a separator andelectrolyte into a sealed cell; c) charging the cell to a voltage abovethat of electrolyte reduction but below that voltage that would violatethe anode maximum safe lithium capacity; d) discharging or partiallydischarging the cell; and e) charging the cell to the normal fullvoltage.
 2. A process as in claim 1, where the cell is discharged andthe partial charge step is repeated, preferably at incrementally highervoltages to stimulate further SEI loss.
 3. A process as in claim 1,where an elevated temperature is applied to the cell after the initialpartial charge for ½ to 7 or more days.
 4. A process as in claim 1,where cell specific capacity and volumetric capacity density areincreased.
 5. A process as in claim 1, where cell capacity retention isincreased.
 6. A process as in claim 1, in which the formation cycles areperformed at elevated temperatures.
 7. A process of maximizing specificcapacity and capacity retention of a lithium ion cell comprising: a)pre-lithiating an anode; b) assembling the anode, a cathode, a separatorand electrolyte into a sealed cell; c) heating the cell to an elevatedtemperature; d) charging the cell to a voltage above that of electrolytereduction but below that voltage that would violate the anode maximumsafe lithium capacity; e) optionally discharging the cell and repeatingthe partial charge step above, possibly at incrementally higher voltagesto stimulate further SEI loss; f) optionally discharging the cell; andg) charging the cell to the normal full voltage.
 8. A process as inclaim 7, where cell specific capacity and volumetric capacity densityare increased.
 9. A process as in claim 7, where cell capacity retentionis increased.
 10. A process as in claim 7, in which an elevated storagestep is used prior to step g.
 11. A process of maximizing specificcapacity and capacity retention of a lithium ion cell comprising: a) ananode; b) assembling the anode, a cathode, a separator and electrolyteinto a sealed cell; c) charging the cell to a voltage above that ofelectrolyte reduction but below that voltage that would violate theanode maximum safe lithium capacity; d) discharging or partiallydischarging the cell; and e) charging the cell to the normal fullvoltage.